Multi-channel communication systems are often susceptible to interference between the various channels, also referred to as crosstalk or inter-channel crosstalk. For example, digital subscriber line (DSL) broadband access systems typically employ discrete multi-tone (DMT) modulation over twisted-pair copper wires. One of the major impairments in such systems is crosstalk between multiple subscriber lines within the same binder or across binders. Thus, signals transmitted over one subscriber line may be coupled into other subscriber lines, leading to interference that can degrade the throughput performance of the system. More generally, a given “victim” channel may experience crosstalk from multiple “disturber” channels, again leading to undesirable interference.
Different techniques have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectrum management techniques to multi-channel signal coordination.
By way of example, certain of the above-noted techniques allow active cancellation of inter-channel crosstalk through the use of a precoder. In DSL systems, the use of a precoder is contemplated to achieve crosstalk cancellation for downstream communications between a central office (CO) or another type of access node (AN) and customer premises equipment (CPE) or other types of network terminals (NTs). It is also possible to implement crosstalk control for upstream communications from the NTs to the AN, using so-called post-compensation techniques implemented by a postcoder.
One known approach to estimating crosstalk coefficients for downstream crosstalk cancellation in a DSL system involves transmitting distinct pilot signals over respective subscriber lines between an AN and respective NTs of the system. Error feedback from the NTs based on the transmitted pilot signals is then used to estimate crosstalk. Other known approaches involve perturbation of precoder coefficients and feedback of signal-to-noise ratio (SNR) or other interference information.
Crosstalk estimates are commonly utilized in situations in which it is necessary to “join” an additional line to a group of active lines in a DSL system. For example, it may become necessary to activate one or more inactive lines in a synchronization group that already includes multiple active lines, where synchronization in this context refers to alignment in time of the DMT symbols for the different lines. Such joining of an additional line may require that the precoder be adjusted accordingly in order to optimize system performance. Crosstalk estimates are also used in other situations, such as tracking changes in crosstalk over time. Thus, crosstalk estimation may be used to determine the residual crosstalk after precoding and this information can be used to adjust the crosstalk coefficients.
Conventional crosstalk reduction techniques are deficient in terms of the information transfer rate required between a given transmitter and the precoder. For example, in certain DSL systems it is known to supply signals to the precoder using an m-bit representation. More specifically, a portion of a data stream that is to be transmitted is first mapped to constellation points and then scaled in the transmitter to obtain a signal, and each signals is sent to the precoder as a sequence of complex values, with m bits being used to represent each of the real and imaginary components of the signal, such that 2m bits are required per signal per tone. Such an arrangement unduly increases the bandwidth requirements of the precoder interface, and limits the throughput performance of the system. Also, the use of the m-bit representation can introduce significant quantization error into the data that is applied to the precoder.
Accordingly, a need exists for improved precoding arrangements that can reduce the bandwidth requirements of the precoder interface while also limiting the adverse impact of quantization error on precoded signals.